How to account for using spacers in a motor case in OpenRocket?

[Chris_H]

Hi all.

I am still relatively new to OpenRocket.


When designing, how does one distinguish between a 4G motor in a 4G case and a 4G motor in a 6G case using spacers?

I have been looking, and cannot see where this is distinguished.

Is it solved through determining the weight of the case and closures, and then adding a mass object? Or how?

 

[dshmel]

Is it solved through determining the weight of the case and closures, and then adding a mass object? Or how?

That is how I would do it. Add a mass object equal to the weight difference between a 6G and 4G case plus the spacers. I guess it depends on how accurate you need to be about the weight for your particular set up and flight. I don't bother adjusting the weight in RS or OR when I use spacers. For me, the added weight is such a small percentage of the total pad ready weight, that it falls into the category of other variables that exist but I cannot control. Variables such as motor to motor performance differences, the exact environmental conditions at the time of launch (temp/humidity and wind) all attribute to the difference between simulated and actual recorded altitude.

I use simulators for rough altitude guidance for selecting motors and for fine tuning delay elements for motor ejection (which I use less frequently now).

 

[DavidMcCann]

Adding a mass object would be the only way I can think of.

(I also have never bothered with it.)

 

[Chris_H]

Cool. Thank you for the replies.

 

[Chris_H]

To hijack my own thread, and in order to avoid starting a new one, At what point does a rocket become over stable? Is 1.6 or 1.9 overstable? And if the answer depends on variables, how?

If the stability is coming in at, say, 2.4 cal., Does that mean that the fin design could be scaled down slightly?

 

[Igotnothing]

"Overstable" doesn't mean "crash". It just means the rocket will wind-vane more. "More" is a highly subjective term. First launch of the morning with no wind = doesn't matter.

How fast will the rocket be? For some definitions of the subjective term "fast", the term "overstable" means "doesn't crash".

2.4 is a good number for anyone who has to ask the question about 2.4. Very different numbers (higher or lower) mean a lot more needs to be known about the rocket and motor.

 

[Chris_H]

4" diameter airframe, 75mm 4 Grain motor. ( CTI L1115) , Est. Mach 1.5, Approx. 75" tall, 3 fin design. Current stability about 1.8 cal.

"Overstable" doesn't mean "crash". It just means the rocket will wind-vane more. "More" is a highly subjective term. First launch of the morning with no wind = doesn't matter.

How fast will the rocket be? For some definitions of the subjective term "fast", the term "overstable" means "doesn't crash".

2.4 is a good number for anyone who has to ask the question about 2.4. Very different numbers (higher or lower) mean a lot more needs to be known about the rocket and motor.

 

[Nytrunner]

To hijack my own thread, and in order to avoid starting a new one, At what point does a rocket become over stable? Is 1.6 or 1.9 overstable? And if the answer depends on variables, how?

If the stability is coming in at, say, 2.4 cal., Does that mean that the fin design could be scaled down slightly?

**Edited since you posted dimensions**. You're probably good with that 7.2" separation, but keep in mind there's a forward shift of the CP with Mach increase. If the CG-CP distance divided by rocket length is more than ~18% you're overstable. Not saying it'll fly terribly, but it will have a much greater probability of nosing into the wind.

Stubbies (small L/D) are stable with a lower "caliber". Super-rocs (High L/D) often need more than 2. Yours has a L/D of 18.75", so that's almost into the higher end of things.

I won't get into my annoyance with "calibers" here lol

 

[Chris_H]

Rough specs in my last post.

CG-CP distance divided by LOA is about 9%.

Using the method you mention, if 18% is around the upper end threshold, where would the lower limits be? given specs similar to what I mentioned above.

Depends. What's the diameter and length of the rocket? If the CG-CP distance divided by rocket length is more than ~18% you're overstable. Not saying it'll fly terribly, but it will have a much greater probability of nosing into the wind.

Stubbies (small L/D) are stable with a lower "caliber". Super-rocs (High L/D) often need more than 2.

I won't get into my annoyance with "calibers" here lol

 

[Nytrunner]

8-15% is the design range I found a year or so ago when I followed a chain of links/resources from a different thread (which I haven't been able to rediscover, to my great annoyance....). >18 definitely overstable. 8-15 ideal depending on flight regimes (if you're planning to go much beyond supersonic, make it higher. <8 is marginal to unstable

I try and keep my personal rockets at ~12. (coincidentally, that 2.4 'calibers' isn't a bad margin for your rocket lol)

 

[Chris_H]

Thank you, Nytrunner.

This is for a couple of identical rockets I will be bringing to Balls this year. There is a pile of almost all materials and hardware necessary to complete these, so the next bit is in checking as best I can, the design. Plenty of time for that, as the next steps are in designing a jig to taper fins with my 6" x 108" oscillating belt sander, and also in building a slotting jig to cut slots for the fins. And in deciding whether I need an aluminum leading edge for the fins, then executing it. If it is an aluminum edge, then designing a jig for my machinist. Thoughts on an aluminum leading edge for Mach 1.5 flights?

 

[Nytrunner]

Aluminum edges. I'm only level 1, so beyond college competitions with L3 motors, I haven't flown at your scale yet (and we kept our speed down). Beyond that, if you're only going M1.5, they shouldn't be necessary from an engineering standpoint. How fast do your simulations predict?

If you're new to OpenRocket, can I assume you ahbent used RasAero II? It seems to be the go to for extreme flight simulation in the hobby community.

Go have a Ball! (at BALLS)
(added question: What's your goal at BALLS? Speed? Altitude? Research motor? Payload experiment? Getting yelled at by old guys?)

 

[Chris_H]

You would laugh if you knew the reality. I am all ears to you, my friend, not so much the other way around, except I am pretty handy when it comes to building and solving building problems. I have been seeking advice from more experienced people, but some things, like this post, I figure that the forum post is more appropriate in order to be respectful of people's time. The playa is a big place, perfect for bigger rockets. It's all a matter of scale, though the learning curve is exponentially more severe and expensive. Most people would not choose a small dinghy for the open ocean.

Sims with the largest motor I am likely to fly, are about Mach 1.4- 1.7. These are with 'L' motors. This design will be as 'future-proofed' as possible, so one or both of them could fly a larger motor someday, if they do not get lost one way or another, which is why the aluminum edged fins, and also for the experience in making them.

Aluminum edges. I'm only level 1, so beyond college competitions with L3 motors, I haven't flown at your scale yet (and we kept our speed down). Beyond that, if you're only going M1.5, they shouldn't be necessary from an engineering standpoint. How fast do your simulations predict?

If you're new to OpenRocket, can I assume you ahbent used RasAero II? It seems to be the go to for extreme flight simulation in the hobby community.

Go have a Ball! (at BALLS)

 

[ChrisAttebery]

You don't need to worry about aero heating until you get into the M2.5-M3 range. Your FG fins will be fine.

1.8 calibers of stability is also perfectly fine.

For reference my 4" Punisher only hit M1.4 on a M1780.

 

[Chris_H]

Thanks, Chris.

You don't need to worry about aero heating until you get into the M2.5-M3 range. Your FG fins will be fine.

1.8 calibers of stability is also perfectly fine.

For reference my 4" Punisher only hit M1.4 on a M1780.

 

[Chris_H]

Actually, to learn about building rockets that fly at arguably one of the coolest places on the planet (that is within reason to travel to) and to fly rockets that go kinda high, and kinda fast, that fly well and survive the flight. To learn from some of the most experienced in the hobby HPR community, 'old guys' and anyone else, alike. Feedback from 'old guys' can be some of the best available, grumpy or not.

Go have a Ball! (at BALLS)
(added question: What's your goal at BALLS? Speed? Altitude? Research motor? Payload experiment? Getting yelled at by old guys?)